Chapter 7 Chemical Quantities 7.5 Molecular Formulas Brightly colored dishes are made of melamine. Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Relating Molecular and Empirical Formulas A molecular formula is equal or a multiple of its empirical formula has a molar mass that is the product of the empirical formula mass multiplied by a small integer molar mass = a small integer empirical mass is obtained by multiplying the subscripts in the empirical formula by the same small integer Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Diagram of Molecular and Empirical Formulas A small integer links a molecular formula and its empirical formula a molar mass and its empirical formula mass Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Some Compounds with Empirical Formula CH2O Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Calculating a Molecular Formula from an Empirical Formula Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Finding the Molecular Formula Determine the molecular formula of a compound that has a molar mass of 78.11 g and an empirical formula of CH. STEP 1 Calculate the empirical formula mass. Empirical formula mass of CH = 13.02 g STEP 2 Divide the molar mass by the empirical formula mass to obtain a small integer. 78.11 g = 5.999 ~ 6 13.02 g Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Finding the Molecular Formula (continued) STEP 3 Multiply the empirical formula by the small integer to obtain the molecular formula. Multiply each subscript in C1H1 by 6. Molecular formula = C1x6 H1x6 = C6H6 Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Learning Check A compound has a molar mass of 176.1g and an empirical formula of C3H4O3. What is its molecular formula? 1) C3H4O3 2) C6H8O6 3) C9H12O9 Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Solution STEP 1 Calculate the empirical formula mass. C3H4O3 = 88.06 g/EF STEP 2 Divide the molar mass by the empirical formula mass to obtain a small integer. 176.1 g (molar mass) = 2 88.06 g (empirical formula mass) STEP 3 Multiply the empirical formula by the small integer to obtain the molecular formula. molecular formula = 2 x empirical formula C3x2H4x2O3x2 = C6H8O6 (2) Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Molecular Formula A compound contains 24.27% C, 4.07% H, and 71.65% Cl. The molar mass is about 99 g. What are the empirical and molecular formulas? Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Solution STEP 1 Calculate the empirical formula mass. 24.27 g C x 1 mol C = 2.021 mol of C 12.01 g C 4.07 g H x 1 mol H = 4.04 mol of H 1.008 g H 71.65 g Cl x 1 mol Cl = 2.021 mol of Cl 35.45 g Cl Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Solution (continued) 2.021 mol C = 1 mol of C 2.021 4.04 mol H = 2 mol of H 2.02 mol Cl = 1 mol of Cl Empirical formula = C1H2Cl1 = CH2Cl Empirical formula mass (EM) CH2Cl = 49.48 g Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Solution (continued) STEP 2 Divide the molar mass by the empirical formula mass to obtain a small integer. Molar mass = 99 g = 2 Empirical formula mass 49.48 g STEP 3 Multiply the empirical formula by the small integer to obtain the molecular formula. 2 x (CH2Cl) C1x2H2x2Cl1x2 = C2H4Cl2 Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Learning Check A compound is 27.4% S, 12.0% N, and 60.6 % Cl. If the compound has a molar mass of 351 g, what is the molecular formula? Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Solution STEP 1 Calculate the empirical formula mass. In 100 g, there are 27.4 g S, 12.0 g N, and 60.6 g Cl. 27.4 g S x 1 mol S = 0.854 mol of S 32.07 g S 12.0 g N x 1 mol N = 0.857 mol of N 14.01 g N 60.6 g Cl x 1mol Cl = 1.71 mol of Cl 35.45 g Cl Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Solution (continued) STEP 2 Divide the molar mass by the empirical formula mass to obtain a small integer. 0.854 mol S = 1.00 mol of S 0.854 0.857 mol N = 1.00 mol of N 1.71 mol Cl = 2.00 mol of Cl empirical formula = SNCl2 empirical formula mass = 116.98 g Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Solution (continued) STEP 3 Multiply the empirical formula by the small integer to obtain the molecular formula. Molar mass = 351 g = 3 Empirical formula mass 116.98 g Molecular formula = (SNCl2)3 = S3N3Cl6 Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Concept Map Basic Chemistry Copyright © 2011 Pearson Education, Inc.